Axon Routing in the Olfactory System

The olfactory system of mice entails a developmental program that wires neurons expressing similar olfactory receptors into glomeruli together. Although the adult olfactory system continues to produce and incorporate new neurons, it cannot withstand severe damage (see the Perspective by Cheetham and Belluscio). Ma et al. (p. 194) and Tsai and Barnea (p. 197) examined the difference in responses between early development and adulthood. Manipulating the expression of an odorant receptor or the activity of the olfactory neurons altered olfactory neuron axonal pathfinding. The results suggest that the guidance systems used differ between early development and adulthood: Early axons find their own way, but later-in-life axons can only follow existing pathways.

Abstract

The olfactory system remains plastic throughout life because of continuous neurogenesis of sensory neurons in the nose and inhibitory interneurons in the olfactory bulb. Here, we reveal that transgenic expression of an odorant receptor has non–cell autonomous effects on axons expressing this receptor from the endogenous gene. Perinatal expression of transgenic odorant receptor causes rerouting of like axons to new glomeruli, whereas expression after the sensory map is established does not lead to rerouting. Further, chemical ablation of the map after rerouting does not restore the normal map, even when the transgenic receptor is no longer expressed. Our results reveal that glomeruli are designated as targets for sensory neurons expressing specific odorant receptors during a critical period in the formation of the olfactory sensory map.